<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.9/vue.js"></script>

<body>
    <div id="app">
        <input type="text" v-model="firstName"><br>
        <input type="text" v-model="lastName"><br>
        <input type="text" v-model="fullName"><br>
    </div>
    <script>
        let vm = new Vue({
            el: '#app',
            data: {
                firstName: 'a',
                lastName: 'b',
                fullName: 'a-b',
                obj: {
                    name: 'aa',
                    age: 21
                }
            },
            watch: {
                firstName(value) {
                    this.fullName = value + '-' + this.lastName;
                },
                lastName(value) {
                    this.fullName = this.firstName + '-' + value;
                },
                fullName(value) {
                    let arr = value.split('-');
                    this.firstName = arr[0];
                    this.lastName = arr[1]
                },
                obj: {
                    handler(value) {
                        console.log(value);
                    },
                    immediate: true
                }
            }
        })
    </script>
</body>

</html>